This project will experimentally determine the onset of phagocytosis in the pigment epithelium (PE) during development and will characterize the phagocytic activity of this tissue in the adult. It will utilize a procedure which allows particulate material to be introduced into the sub-retinal space where it is available for uptake or exlusion by the PE. Assessment of whether the test material had been phagocytized will be judged cytologically with light and electron microscopic methods. It will determine when the PE in the chick embryo develops its phagocytic capability. It will determine whether the PE in the newborn rat is capable of phagocytosis or whether phagocytosis commences at the time outer segment fragments are first observed in this tissue. It will determine the relationship between particle size and uptake by the PE in the rat. It will determine whether the PE in the dystrophic rat is capable of phagocytosis. Using the frog as an assay organism, it will determine surface characteristics required of particulate material necessary to elicit uptake by the PE.